1. Field of the Invention
This invention relates to a technology for detecting misfire occurring during operation of a gasoline or other spark-ignition internal combustion engine and more particularly to a spark plug cap with a capacitor for detecting misfire for a spark-ignition internal combustion engine.
2. Description of the Prior Art
As is well known, in gasoline and other types of internal combustion engines a high voltage produced by an ignition coil is distributed to spark plug at the engine cylinders by an ignition distributor or the like. At each cylinder, the resulting electric discharge between the spark plug electrodes produces a spark which ignites an air-fuel mixture that has been drawn into the cylinder and compressed at the proper time, causing the mixture to burn explosively. In the course of this ignition-combustion process in the internal combustion engine, the mixture may for some reason occasionally fail to burn properly. This is referred to as a misfire. Misfires can result from causes in either the fuel system or the ignition system. Misfires caused by problems in the fuel system are the result of an overly lean or overly rich air-fuel mixture. A spark is produced between the spark plug electrodes but the air-fuel mixture does not ignite. Misfires caused by problems in the ignition systems are the result of spark plug electrodes fouling or ignition circuit malfunctions which prevent normal spark discharge.
The occurrence of misfire in the course of engine operation not only degrades engine performance but may also cause after-firing of unburned gases in the exhaust system, which can damage the exhaust gas aftertreatment system and have other adverse effects. Moreover, since the occurrence of even a single misfire indicates a misadjustment or malfunction in the fuel or ignition system, prompt elimination of the problem is essential. Because of this, there is a strong need for development of a detector for detecting misfires as soon as they occur.
One type of misfire detector that has been proposed is the mis-spark detector described in Japanese Laid-open Patent Publication No. 52(1977)-118135. As shown in FIG. 7, the detector includes a conductor 51 wrapped around a portion of a high-voltage (high tension) cable 50 of the engine ignition system so as to constitute a detection capacitor 52 (a type of capacitance probe) in which the insulation cladding 50A of the high-voltage cable 50 serves as the dielectric. A voltage divider capacitor 53 is connected between the capacitor 52 and ground so that the ignition voltage (secondary voltage of the ignition coil) applied to the conductive core 50B of the high-voltage cable 50 induces a voltage across the terminals of the capacitor 52 owing to its static capacitance. The induced voltage is statically divided by the capacitor 52 and the capacitor 53, and the voltage across the terminals of the capacitor 53 (the divided voltage) is forwarded as a detection voltage to an electronic circuit 54 for processing and discrimination. The electronic circuit 54 discriminates the occurrence of misfires from the difference between the wave form of the ignition voltage at the time of normal spark discharge and that at the time of no spark discharge (mis-sparking). Among the different types of misfires, the detector thus detects misfires that occur when no spark discharge is produced owing to a problem in the ignition system.
Another detector for detecting misfire in internal combustion engines is disclosed in the present assignee's Japanese Patent Application No. 3(1991)-326509. In this detector, the ignition voltage is similarly detected from a high-voltage cable or the like of the ignition system using static voltage division, and misfiring owing to causes in the fuel system is detected based on the fact that, even when spark discharge occurs, the wave form of the ignition voltage differs between the case where normal combustion occurs and the case where it does not.
In the conventional misfire detectors, as the means for detecting ignition voltage there is usually used a so-called capacitance probe constituted by wrapping a sheet or ribbon of conductor around the high-voltage cable of the ignition system so as to form a detection capacitor between the conductor and the core of the high-voltage cable via the insulation cladding of the high-voltage cable as the dielectric. However, the capacitance probe constituted in this manner has a major drawback that derives from the nature of the high-voltage cable of the ignition system. Because of its flexibility and elasticity, the high-voltage cable is highly susceptible to vibration. It is also easily affected by changes in the ambient humidity, wetting by leaking water, and fouling with oil, grime and the like. When a capacitor for use in detection is formed by wrapping a conductor ribbon around the cable, the static capacitance of the capacitor is apt to be changed from the proper value by shifting of the conductor caused by vibration as well as by changes in humidity, wetting with water and fouling with oil, grime and the like. Although some change in static capacitance can be tolerated if the capacitor is to be used only for checking the ignition voltage, even slight changes have to be avoided when it is used for misfire discrimination because such discrimination generally requires accurate detection not only of the ignition voltage but also of the ignition voltage wave form. The capacitance changes to which the prior art capacitance probe is susceptible degrade the detected voltage wave form and make it impossible to discriminate misfire with high reliability.
In actual practice, moreover, the flexibility and elasticity of the high-voltage cable make the work of attaching the conductor for forming the capacitor on the insulation cladding of the high-voltage cable difficult and troublesome. Maintenance of the so-formed capacitor is also troublesome.
In addition, the insulation cladding of the high-voltage cable is generally formed of synthetic rubber, a material that is readily degraded when exposed to heat and fouling with oil and grime. This degradation of the insulation cladding after the detection capacitor has been formed by winding the conductor around the high-voltage cord not only produces a progressive change in the static capacitance of the capacitor over time but may also reduce the electric insulation property of the cladding to the point that the high ignition voltage can leak to the conductor wrapped thereon. When this happens, the high leak voltage is apt to be conducted to the electronic circuitry of the misfire detector, which it can damage or cause to malfunction. Ideally, moreover, for enabling accurate detection of the ignition voltage wave form, the distance between the conductor and the conductive core constituting the opposite sides of the detection capacitor should be made as small as possible so as to increase its static capacitance. In actuality, however, the conductor and the cable conductive core have to be spaced at a greater distance than desirable as a precaution against loss of the insulation property by the cable cladding owing to the aforesaid causes as well as to corona discharge. As a result, it has been impossible to avoid low accuracy in detecting the ignition voltage wave form.